Don't you love spring, where the threads slowly transform from "my ESOF doesn't work" to "wow, this thing sucks fuel like a sailor swilling run while on shore leave".

The 6.4L is a very good engine. It's weakness if fuel mileage. The reason it does have low fuel mileage is because of the pollution control equipment needed to meet the stringent EPA requirements.

6.4L owners have had great success boosting the fuel mileage by using a DPF delete strategy coupled with some sort of tune, but please be painfully aware that any engine modification WILL void your engine warranty.

If you have a Job#1 truck, you can install the front chin valance from a Job#2 or newer truck. The lip is deeper, which helps divert air from under the truck and will boost fuel mileage a little.